This invention relates in general to high-temperature vacuum furnaces in which, for example, ferrous workpieces may be brazed, or surface hardened, or otherwise treated, under reduced pressure. The reduced pressure may range from sub-atmospheric pressure to high vacuum.
The invention relates particularly to means for sensing the temperatures at the surfaces of a number of workpieces as they are conveyed on a carrier through the vacuum furnace and for transmitting with high accuracy the sensed temperature data to a telemetry receiver located outside the furnace.
A specific application of the invention will be described. However, it is to be understood that the invention has other applications and may be used for other purposes than that which is specifically described. The scope of the invention is, of course, defined in the claims which are appended to this application.
One specific use of the invention which will now be described is in connection with the brazing of small radiators of the type which are used in automobiles for various purposes, such as the heating of the interior of the automobile, cooling of the transmission fluid, and other purposes. These radiators are considerably smaller than the radiator which is used to cool the engine. They are, however, similar in that they comprise a series of tubes which are connected by fins. Connection of the fins to the tubes may be accomplished by brazing, i.e., by subjecting the radiators to high temperature in a vacuum furnace.
In a typical installation, a platform or tray type of carrier is used to carry a quantity of these small radiators through a vacuum furnace on a conveyor mechanism. The same carrier also carries a telemetry box which houses a telemetry system whose function is to detect the temperatures on the surfaces of a number of the radiators, for example, on the surfaces of fifteen radiators spaced apart and randomly selected, and to transmit accurate temperature data to a receiver located outside of the vacuum furnace. The data received is then processed in a computer and, if necessary, adjustments are made by the computer, or manually, to one or more of the heating elements of the vacuum furnace for the purpose of achieving uniform brazing of the radiators.
The telemetry system includes a plurality of thermocouples whose dissimilar metal junctions or tips are positioned on the variously positioned, randomly selected, radiators. The temperature data sensed by the thermocouple tips is carried by the thermocouple conductors into the interior of a heat-insulated box which houses the telemetry chassis. Transmitters within the box mounted on the telemetry chassis transmit signals to a receiving antenna mounted on an interior wall of the vacuum furnace and the received signals are carried through the furnace wall to a receiver outside the furnace and then to a computer which controls the system.
In a typical installation, a vacuum furnace comprises three compartments as follows: (1) a preheat chamber; (2) a braze chamber; and (3) an exit chamber. The carrier, which in the present example carries a quantity of small radiators and also the telemetry box, is conveyed by a conveyor to the entrance door of the preheat chamber. The pressure in the chamber is raised to atmosphere and the door is opened. The carrier is conveyed within the preheat chamber, the entrance door is closed, and the preheat chamber is pumped down to a vacuum pressure while the work is preheated. After a period time, the door separating the preheat chamber from the inward braze chamber is opened and the carrier with its radiators and telemetry box is transported by the conveyor into the braze chamber which is at a higher vacuum. The door between the preheat and braze chambers is then closed, and the workpieces (radiators) are subjected to a temperature profile, one example of which will be described in detail later on in this patent application. After a preselected time period, the door which separates the braze chamber from the exit chamber is opened and the workpieces are transported into the exit chamber. The door between the braze and exit chambers is closed. The pressure in the exit chamber then rises to atmosphere. The exit door is opened and the conveyor then carries the carrier with the workpieces and telemetry box thereon to a cooling chamber where the radiators and the telemetry box are rapidly cooled. The conveyor next carries the carrier to an unloading station where the cooled radiators are removed. The carrier carrying the telemetry box is then conveyed to a loading station where the carrier is reloaded, and the cycle just described is repeated.
As is known, a thermocouple comprises a dissimilar metal junction which is located at the tip and from which extends a pair of very fine dissimilar wires. In at least some of the prior art installations of the type here involved, and described above, the very fine dissimilar wires of the thermocouple conductors are terminated in an electrical plug which is inserted into a socket provided in a wall of the telemetry box. Another pair of dissimilar wires extends from the socket to the transmitters within the telemetry box. The transmitters are powered by batteries mounted on a chassis contained within the telemetry box. In prior art installations of which applicant is aware, these batteries are ON for substantially the entire work cycle described above.